Self-etching dental adhesives of the state of the art typically contain one or more acidic monomers, solvents, initiators, stabilizers, and fillers. Fillers are typically added for at least two reasons:
Firstly, fillers are sometimes added for reinforcing the cured adhesive. Fillers which are added for this purpose usually comprise ground glass, quartz or other minerals, pyrogenic (fumed) or precipitated colloidal silica. All of these fillers are typically surface-treated with a functional coupling agent. This coupling agent is applied to the filler in an extra step.
In this respect, EP 1 101 484 A (Shofu) discloses an inorganic filler that comprises inorganic fine particles, wherein the surface of the inorganic fine particle is covered with polysiloxane.
EP 1 532 958 A1 (GC) discloses inorganic fine particles having 1-100 nm average particle diameter and surfaces modified with alkoxysilane having an unsaturated double bond which are contained in a (meth)acrylate monomer in the monodispersed state.
EP 0 803 240 B1 (Ivoclar) describes a sol of SiO2 particles in a liquid, organic dispersion agent, the SiO2 particles being organically surface-modified, having an average size of 10 to 100 nm and being non-agglomerated.
WO 99/17716 A1 (Dentsply) describes a low-viscosity dental material containing a non-settling nanoscale filler.
Secondly, fillers are sometimes added as viscosity modifiers. Typically, fumed or pyrogenic silica without surface treatment are employed for this purpose.
In this respect, e.g. EP 1 287 805 A1 (GC) describes a one-pack type dental adhesive composition comprising a viscosity modifier with a primary particle size of 0.01-0.05 μm.
To compatibilize a filler with a monomer matrix, the surface of the filler is usually treated with a functionalized silane, involving typically several of the following steps:                dispersing the filler in a solvent        adjustment of the pH        adding of the silane        heat treatment        removal of solvent        drying of the filler        solvent exchange        milling of the filler        re-dispersing of the silane-treated filler in the adhesive        
Each of these steps can be subject to errors, and some require costly equipment as reaction vessels, mills or vacuum pumps.
The addition of large amounts of untreated silica filler to a monomer matrix, however, might lead to unwanted thickening of the formulation causing undesirable handling properties.